Urashima Shu-Hei, Myalitsin Anton, Nihonyanagi Satoshi, Tahara Tahei
Molecular Spectroscopy Laboratory , RIKEN , 2-1 Hirosawa , Wako , Saitama 351-0198 , Japan.
Ultrafast Spectroscopy Research Team , RIKEN Center for Advanced Photonics (RAP) , 2-1 Hirosawa , Wako , Saitama 351-0198 , Japan.
J Phys Chem Lett. 2018 Jul 19;9(14):4109-4114. doi: 10.1021/acs.jpclett.8b01650. Epub 2018 Jul 11.
Despite recent significant advances in interface-selective nonlinear spectroscopy, the topmost water structure at a charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ spectrum. The obtained Imχ spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 to 2 M.
尽管界面选择性非线性光谱学最近取得了重大进展，但带电二氧化硅表面的最上层水结构仍未得到清晰的理解。这是因为，对于带电界面，即使使用二阶非线性光谱学，不仅会探测最上层的界面分子，还会探测双电层中的大量分子。在本研究中，我们使用外差检测振动和频产生光谱学研究了pH值为12时带负电的二氧化硅/水界面的水结构，并证明通过检查Imχ光谱的离子强度依赖性可以提取最上层水的光谱成分。获得的Imχ光谱表明，最上层的主要水物种通过一个OH基团与二氧化硅表面带负电的硅醇盐形成氢键。还存在少量水物种，它们使用一个OH基团与硅氧烷桥的氧原子或二氧化硅表面剩余的硅醇发生弱相互作用。Imχ光谱的离子强度依赖性表明，最上层的这种水结构在0.01至2 M的宽离子强度范围内保持不变。
